At the beginning of any secure network design project, many best practices apply more or less uniformly to all areas of the design. This article by Sean Convery presents these practices in a single location.

Many things difficult to design prove easy to performance. Samuel
Johnson, Rasselas: The History of Rasselas, Prince of Abissinia, 1759

A good scientist is a person with original ideas. A good engineer is a
person who makes a design that works with as few original ideas as possible.
There are no prima donnas in engineering. Freeman Dyson, Physicist,
Disturbing the Universe, 1979

At the beginning of any secure network design project, many best practices
apply more or less uniformly to all areas of the design. This chapter presents
these practices in a single location and then draws on them throughout the rest
of the book. The designs presented in Chapter 13, "Edge Security
Design," Chapter 14, "Campus Security Design," and Chapter 15,
"Teleworker Security Design," are based on many of the concepts
described here and in the companion chapters (Chapters 711), which detail
specific design considerations for certain technologies. The topics are
presented in loose compliance with the seven-layer OSI model and, as such, cover
a diverse set of topics. Chapter 1, "Network Security Axioms,"
presented the security axioms; this chapter translates them into actionable
guidance for secure network design.

Physical Security Issues

One common security truism is "Once you have physical access to a box,
all bets are off." This is a good beginning assumption for this section. If
an attacker has physical access to a computer, router, switch, firewall, or
other device, your security options are amazingly limited. Networking devices,
with few exceptions, can have their passwords reset by attaching to their
console port. Hosts can be booted with a special floppy disk or CD-ROM designed
to circumvent most host security on the device.

This book does not cover physical security issues in detail. Topics such as
disaster recovery, site selection, and so on are not discussed at all. However,
as a network designer, you must know where you are relying on physical security
to augment or support your network security. There are some rules you can follow
to improve your security:

Control physical access to facilities.

Control physical access to data centers.

Separate identity mechanisms for insecure locations.

Prevent password-recovery mechanisms in insecure locations.

Be aware of cable plant issues.

Be aware of electromagnetic radiation.

Be aware of physical PC security threats.

The rest of this section examines these seven areas.

Control Physical Access to Facilities

Effectively controlling physical access to your organization's
facilities should be the single top concern for both your physical security
staff and you, the network designer. Most organizations utilize one of three
mechanisms to implement physical security (presented in increasing order of
security):

Lock-and-key access

Key card access

Key card access with turnstile

Lock-and-Key Access

The most common physical security control, particularly in smaller
organizations, is traditional lock-and-key access. For this method, individuals
who need access to certain rooms or buildings are given keys for access. This
option has the following benefits:

Generally, this is the cheapest option for small organizations.

No technical experience is required.

Special keys are available to thwart key duplication.

However, there are also several drawbacks:

If employees leave the company on less than amicable terms, they might
"lose" their keys or might simply stop showing up for work. In such
cases, it can be very costly to rekey the locks and redistribute keys to the
valid employees.

Unless coupled with an alarm system that augments the lock-and-key
access, there is no mechanism to determine when employees with keys access a
given physical location.

Most keys can be easily duplicated at the local hardware store.

Key authentication is single-factor, meaning the key is all a
person needs to access locked areas.

Key Card Access

More common in larger organizations, key card access can alleviate some of
the management problems associated with lock-and-key access and can provide
increased security measures. Key card access can take the form of a magnetic
card reader or a smart card. All of these systems have the same basic pros and
cons once you eliminate the technical differences of the technology. These are
the benefits of a key card system:

Access to multiple locations can be controlled with a single
card.

In the event that an employee leaves the company, the employee's
card can be quickly disabled whether or not it is physically returned.

Locks should never need to be "rekeyed."

Facilities with multiple entrances are easily supported.

Reports can be run to show when individuals entered specific
locations.

The drawbacks to a key card system are as follows:

Like lock-and-key access, key cards are single-factor security. Any
individual with a valid key card could access the location.

Key card systems can be expensive, and in the event of a failure in the
central authentication system, all users can be denied access to a
facility.

The principal problem with key card access is tailgating.
Tailgating is gaining unauthorized access to a building by following an
individual with valid access. Oftentimes, if attackers are dressed in the
appropriate clothing, they can simply follow legitimate individuals into a
building without having to present a key card. Even if someone requests to see a
card, an attacker can show an invalid card because it might not actually be
scanned by the card reader.

Key Card Access with Turnstile

Although most often associated with ballparks and stadiums, turnstile access
with a key card can be one of the most secure methods of controlling physical
access to a building. For this method, a key card is used to activate the
turnstile and allow one person into the building. These systems are most common
in large multifloor buildings, where access can be controlled at the ground
floor. In the following list, you can see that this option has all the benefits
of the previous option plus more.

Tailgating is greatly diminished because only one person can enter per
card.

Access to multiple locations can be controlled with a single
card.

In the event that an employee leaves the company, the employee's
card can be quickly disabled whether or not it is physically returned.

Locks should never need to be "rekeyed."

Reports can be run to show when individuals enter specific
locations.

The drawbacks of a system such as this are as follows:

Like the previous two systems, key card access with turnstile is a
single-factor identity system. Any individual with a valid card could gain
access to the building.

This doesn't work well for facilities with multiple buildings and
multiple entrances.

This method generally requires a security guard to verify that
individuals are not hopping over the turnstile or tailgating through an entrance
designed for persons with physical disabilities that bypasses the
turnstile.

Turnstiles are not aesthetically pleasing.

Turnstile access can be inconvenient for employees, escorted guests, or
individuals using dollies for equipment.

This method is more expensive than simple key card access and also has
the same issues in the event of a failure in the key card authentication
system.

Solving the Single-Factor Identity Problem

A second factor can be added to either of the previous key card
authentication processes. The first option is to put a personal identification
number (PIN) code reader at every location where there is a card reader. After
using their key card, employees must enter a PIN to unlock the door. Another
option is to use some form of biometric authentication. Biometric authentication
could be used as either the second factor in a key card system or the principal
factor in a biometric system. In the second case, users would enter a PIN after
successful biometric authentication. See Chapter 4, "Network Security
Technologies," for the pros and cons of biometric authentication. Both of
these alternatives add cost to the system and inconvenience for users.

Control Physical Access to Data Centers

Data-center access can utilize any of the preceding mechanisms in addition to
PIN-reader-only access. The important difference with data-center access is that
you are often dealing with a smaller set of operators, so issues around key
management are somewhat reduced.

NOTE

I once had the pleasure of experiencing a physical security audit by a client
who was considering using a facility in one of my previous jobs. Needless to
say, it didn't go well. One of the auditors was able to gain access to the
building by tailgating. Upon entering, he asked to see the "secure"
data center we had advertised. Upon reaching the entrance to the secure room, he
stood on a chair and pushed up the ceiling tile outside the room. He discovered
that the walls to our data center extended only 12 inches beyond the ceiling
tiles, allowing access if someone climbed over them.

In the context of this discussion, data center refers to any location
where centralized network resources are stored. This could include traditional
data centers, wiring closets, coat closets, or someone's desk. It all
depends on the size of the facility and the way it is organized.

Separate Identity Mechanisms for Insecure Locations

Although identity design considerations are discussed in more detail in
Chapter 9, "Identity Design Considerations," from a physical security
perspective, it is important to ensure that passwords in physically insecure
locations are not the same as those used in secure locations.

Often an organization will utilize common authentication mechanisms for the
various systems that must access network resources. For example, SNMP community
strings or Telnet/ SSH passwords might be set the same on all devices. From a
pure security perspective, it is preferable to use two-factor authentication,
when available, for each user who accesses the network device. Although this
might be possible for users, it is often impossible for software management
systems, which need to run scripts to make changes on several machines at once.
For optimal security, different passwords should be used on each device, but
this is often operationally impossible for large networks.

Therefore, at a minimum, organize your common passwords so that they are
never used on systems in physically insecure locations. For example, assume you
have 3 main locations (with data centers) to your organization and 10 remote
sites (considered insecure). In this case, only use your shared passwords on the
main sites and ensure that the passwords for each of the remote systems are
unique per site at a minimum and per device ideally. As the number of insecure
locations increases into the hundreds or thousands, this becomes impossible;
refer to the "Business Needs" section of Chapter 2, "Security
Policy and Operations Life Cycle," for guidance on calculating the costs
and benefits of this and any other difficult security measure. (People generally
don't compute cost/benefit on easy and cheap security measures.)

Prevent Password Recovery Mechanisms in Insecure Locations

Some devices have controls to prevent the recovery of passwords in the event
that an attacker has physical access to your system. For example, on some newer
Cisco routers and switches, the command is as follows:

Router(config)# no service password-recovery

When this command is entered on a router or a switch, interrupting the boot
process only allows the user to reset the system to its factory default
configuration. Without this command, the attacker could clear the password and
have access to the original configuration. This is important because the
original configuration might contain common passwords or community strings that
would allow the attacker to go after other systems.

This would be particularly useful in insecure branch offices or other
locations where the physical security of a network device cannot be assured.

Be Aware of Cable Plant Issues

In today's networks, there are two primary cable types: unshielded
twisted pair (UTP) category 5 (or higher) and fiber optic. The risk of an
attacker accessing your physical cabling is important to consider because that
level of access often can bypass other security controls and provide the
attacker with easy access to information (provided encryption is not used). UTP
cable is very easy to tap, but it was thought years ago that fiber was immune to
cable taps. We now know that this is not the case. The National Security
Association (NSA) is rumored to have already tapped intercontinental network
links by splicing into the cable; read about it at the following URL:
http://zdnet.com.com/2100-11-529826.html.

It is also theorized that fiber cable could be bent far enough so that some
light would escape if the outer layer of the cable is removed. With the right
types of equipment, this information could then be read.

Additionally, if an attacker gains physical access to a wiring closet or the
fiber cable as it runs in a cable tray above a drop ceiling, tapping the cable
by installing couplers is another possibility.

All this being said, fiber is more secure than copper because the means to
tap the signal are more expensive, difficult to execute, and often require
interrupting the original flow of data to install. On the other hand, the means
to tap a UTP signal can easily be purchased off of the Internet.

Be Aware of Electromagnetic Radiation

In 1985, the concerns of the paranoid among the security community were
confirmed. Wim van Eck released a paper confirming that a well-resourced
attacker can read the output of a cathode-ray tube (CRT) computer monitor by
measuring the electromagnetic radiation (EMR) produced by the device. This
isn't particularly easy to do, but it is by no means impossible. Wim's
paper can be found here:

This form of attack is now commonly called van Eck phreaking.
Additionally, in 2002, Markus Kuhn at the University of Cambridge published a
similar method of reading data off of a CRT, this time by measuring the changes
in the amount of light in a room. His paper can be found here:

I am certainly not recommending that all systems must address these sorts of
security considerations, but it is good to know that such attacks are
possible.

Be Aware of Physical PC Security Threats

Oftentimes, inexperienced network designers begin with an unacknowledged
assumption that all the sensitive data within an organization is
contained on servers. In reality, there is sensitive information about my
company sitting on the laptop I am using to write this book, as well as on the
servers. Like most employees at my company, server resources are used when
necessary, but often interesting information is stored locally.

Several physical security issues manifest when you operate under the
preceding assumption:

The first is that portable computer theft is a big problem, not just in
the cost of replacing the computer but in the proprietary information that is
stored on it. The best protection against having a lost portable computer turn
into lost trade secrets is some type of file system encryption. (Some are built
into modern OSs.) Chapter 4 has more details on such systems.

The second is that by compromising the data coming into and out of a PC,
you can learn passwords, sensitive data, and so on. An attacker can achieve this
through network sniffing, EMR emissions (discussed in the previous section),
remote control software (Back Orifice 2000), or novel devices that attach
between the keyboard and the PC and record to flash memory every key typed. For
more information see this URL: